1. Field of the Invention
This invention relates to a digital image sender, a digital image receiver, a digital image transmission system and a digital image transmission method which allow long-haul transmission of a digital image signal.
2. Description of the Related Art
In recent years, the DVI (Digital Visual Interface) standards used principally in computers and the HDMI (High Definition Multimedia Interface) standards which define additional functions for home appliances based on the DVI standards have been proposed for the transmission of a digital image signal.
Digital transmission based on the interface standards mentioned decreases fluctuation and blurring of an image, inaccuracy in color development and so forth, which have been subjects of existing analog transmission to be solved. However, where such digital transmission is implemented using, for example, metal wires such as a coaxial cable, the distance over which a digital image signal can be transmitted while the quality thereof is maintained is approximately 5 to 10 m. In order to solve this problem, digital video signal interface modules which use an optical fiber only for the transmission of parallel digital image signals and a reference clock signal of a comparatively high speed have been proposed. One of such digital video signal interface modules is disclosed, for example, in Japanese Patent Laid-Open No. 2002-366340 (hereinafter referred to as Patent Document 1).
FIG. 9 shows an example of a configuration of such a digital video signal interface module as disclosed in Patent Document 1. Referring to FIG. 9, the digital video signal interface module shown includes a computer 401, a sender connector 433, optical fibers 437, a receiver connector 435, an LCD monitor 402, and metal wires 440 to 444.
The sender connector 433 includes four laser diodes 438 for transmitting four optical signals including R, G and B digital image signals and a reference clock signal, and a laser driver 430 for driving the laser diodes 438. The receiver connector 435 includes four photodiodes 439 for receiving the four optical signals and a PD (photodiode) amplifier 432 for driving the photodiodes 439.
R, G and B digital image signals and a reference clock signal are outputted from the computer 401 and electro-optically converted from electric signals into optical signals for individual channels by the laser driver 430 and the laser diodes 438 of the sender connector 433. Then, the optical signals are transmitted for the individual channels by the optical fibers 437.
The transmitted signals are opto-electrically converted from optical signals into electric signals for the individual channels by the photodiodes 439 and the PD amplifier 432 of the receiver connector 435 and then inputted to the LCD monitor 402.
On the other hand, parallel control signals such as Vcc, Ground, DDC DATA, DDC CLOCK and HPD signals are transmitted in parallel by the metal wires 440 to 444, respectively. The digital video signal interface module is configured in this manner.
FIG. 10 shows a cross section of a composite cable 450 which is used in the digital video signal interface module described above.
Referring to FIG. 10, the composite cable 450 shown includes optical fibers 437, a power supply line 440, a grounding line 441, a DDC data line 442, a DDC clock line 443, and an HPD line 444. The R, G and B digital image signals and the reference clock signal mentioned hereinabove are optically transmitted through the four optical fibers 437 while the five parallel control signals mentioned hereinabove are electrically transmitted by the five metal wires 440 to 444. In the composite cable 450, since electromagnetic interference (EMI) from the metal wires 440 to 444 makes a problem, a coating is applied to each metal wire in order to reduce the EMI.
The interface module described above with reference to FIG. 9 uses such a composite cable 450 as described above with reference to FIG. 10 to transmit the R, G and B digital image signals and the reference clock signal using four optical fibers thereby to implement long-haul transmission of a digital video signal.
Meanwhile, a digital image communication apparatus is disclosed in Japanese Patent Laid-Open No. 2005-73220 (hereinafter referred to as Patent Document 2). According to the digital image communication apparatus disclosed in Patent Document 2, a digital image signal which includes parallel digital image signals at least including RGB image signals and a reference clock signal is transmitted in the following manner. In particular, a carrier clock signal is produced based on the reference clock signal and is used to convert the parallel digital image signals at least including the RGB image signals into a serial digital signal. Then, the serial digital signal is converted into and transmitted as an optical signal. Where such an apparatus configuration as just described is adopted, the R, G and B digital image signals and the reference clock signal can be transmitted by a single optical fiber. Therefore, the number of optical fibers can be reduced.